The present invention relates to evaluation of gemstones and other transparent specimens and in particular to color grading of gemstones.
The commercial value of a gemstone depends upon the gemstone weight, cut, clarity and color. Highly trained specialists, known as graduate gemologists evaluate these and other stone properties such as finish to asses a value or grade for the stone. Small changes in any of these evaluation parameters can translate into a large change in stone price. For example, there has been a large increase in colored diamond traffic in recent years. Color, more than anything else, determines the value of a colored diamond. A small change of hue can mean a price change of thousands of dollars. Because of the sensitivity of gemstone value to grading criteria, in particular color, consistent and reliable grading of gemstones is of particular importance to the industry.
Certain of the evaluation criteria lend themselves readily to objective measure. For example, stone weight is measured in units of carats where five carats equal one gram. Clarity describes a stone's internal imperfections other than cleavage or fracture. Although the impact of imperfections on the stone's commercial value may vary slightly from gemologist to gemologist, the imperfections, such as included minerals and trapped gas, are readily visible by inspection and easily described in a standardized language. Gem color, however, does not as readily lend itself to uniform description among gemologists. Gemstone color remains a difficult property to assess and describe based on visual perception of the stone color because a variety of environmental, physiological and cultural factors combine to uniquely influence the gemologist's perception of stone color.
Environmental factors influencing color perception include the lighting conditions under which the stone is viewed. The stone's perceived color is a function of light temperature and spectrum. Daylight Kelvin temperature fluctuates dramatically from about 2000.degree. K. to 25000.degree. K. Because of this wide fluctuation, gem dealers buying stones in Thailand under a hot sun are frequently disappointed in the gem colors upon return to the United States. Lighting sources also emphasize certain colors and de-emphasize other colors depending upon the spectral emissions of the light source itself. For these reasons, a gemstone viewed under two different light sources may produce two different impressions of gem color.
Physiological factors also influence the gemologists color perception. Physiological factors derive from the limitations of the human eye as a measuring instrument and the unique variations of that measuring instrument between individuals. The human eye is limited in the degree to which it can distinguish subtle gradations in color. Furthermore, the wavelength of light at which the color receptors of the eye have maximum sensitivity vary from individual to individual. This variation also results in different color perceptions among individuals.
Another physiological limitation is the capacity of the human brain to interpret and process color. For example, the human brain cannot "remember" a given color. On a day to day basis, the "remembered" color shifts in a random direction. Hence, although the human brain can match two stones by direct comparison, the human brain cannot match two colored stones from memory. This limitation is of particular concern when choosing and matching gemstones for jewelry (i.e., a pair of earrings).
In addition to physiological factors, cultural influences also affect the gemologist's perception of a gem color. In certain societies, color plays a more important role than in other societies. Persons in color sensitive cultures learn a color sensitivity and develop a color memory not shared by persons in other cultures. Some cultures also emphasize or devalue certain colors. Persons in those cultures may develop a skewed sense of color perception.
Even if all persons viewing the stone were to perceive the same color, there remains the problem of describing the color in some universally understood terminology. Gemologists evaluate and describe the stone's color using techniques and terminology promulgated by standards organizations. In the United States, the Gemological Institute of America (GIA) promulgates these standards. Under the GIA guidelines, gemologists describe the stone's predominant color using three parameters: hue, tone, and saturation. Hue is an attribute that describes how the stone appears to be similar to one, or to proportions of two, perceived colors: red, yellow, orange, green, blue and purple. Tone represents the lightness or darkness of hue. Standardized grading schemes structure tone according to a grading scale, having approximately ten divisions, ranging from light to dark. The light end of the scale is colorless and the dark end of the scale is black. Middle grapy occupies the central position. Tone represents the color saturation. More intense colors have less neutral gray mixed with the hue. Dull intensities have greater amounts of neutral gray mixed with the hue.
Typically, the gemologist judges these parameters by matching the stone's color to standardized color pallets representing various combinations of hue, tone and saturation. However, due to the thousands of possible combinations, and the inaccuracies of the human eye, only a limited subset of the universe of colors are represented in the standardized pallets.
One further attribute of gemstones complicates color grading. In most objects, color depends upon what wavelength of light reflects off the surface of the sample. Unlike most other objects of color, gemstone color depends upon light that travels through the stone and is either bounced back by the internal facets or is otherwise refracted by the stone. The multiple paths through the stone, give rise to perception of secondary colors in addition to the primary of dominant stone color. These secondary colors must also be evaluated and described by the gemologist.
Known work sheets are used for color grading which include recording of the gemologists personal evaluation of various significant value factors: including dominant and secondary colors, with comments on clarity information and various cut information. The worksheet information for a number of stones is typically assembled by a gem wholesaler and circulated to various potential purchasers in catalog form. Purchasers can then select stones for potential acquisition from this catalog.
Because the color grading and valuation of gemstones is complicated by the individualism of the gemologist's color perception and by the limitations of the descriptive terminology, the purchaser has limited confidence in the catalog stone grading information. The color picture is also subject to inconsistencies due to the lighting conditions, the chemical properties of the film and errors introduced in the reproductive process. For these reasons, a purchaser is seldom willing to rely solely on the written grading information when making a purchase. The purchaser typically arranges to have the stone shipped to him for inspection and can then verify that the stone meets his appraisal standards and his particular needs.
If the stone does not satisfy the prospective purchaser, the purchaser returns the stone and the dealer can then provide the stone to another prospective customer for inspection. Typically, a stone must be shipped to approximately twelve potential customers before finally being purchased. Each potential purchaser keeps the stone an average of one month. Thus, the typical stone circulates around the world for a period of sixteen months.(counting transportation time) before being purchased. This trading system consumes time and places the stone at risk of loss.
Reduced confidence in the grading not only results in economic inefficiencies in the gem industry itself but also complicates governmental functions and support industry operations. For example, if a stone description cannot be given with confidence, the police tasks of locating and verifying a stolen stone become difficult. Uncertain stone descriptions also cloud the underwriting of gemstones.
The current system of buying and selling gems can be improved if more standardization exists in the color grading of gemstones. With greater standardization and less subjectivity, the gem purchaser can place more confidence in the appraisal and description of the stone.
In addition, forced consistency in dealer's appraisals of stones would require the dealer to be up front about the stone's value, providing improved protection to both consumers and customs operations. Currently, stones are routinely transferred between countries at 1/10 of actual value to avoid duties and other taxes. A reliable and computerized valuation will contribute significantly to curb this practice.
Prior art that is relevant to gemstone evaluation includes U.S. Pat. No. 5,164,586 issued Nov. 17, 1992 to Hohberg. In this system, two point detectors are utilized to obtain a reference value and a color value for the subject. This system utilizes an integrating sphere with an internal light source and multiple light exit points. The reference point is located inside the sphere of the Hohberg patent and is influenced by the object color. The Hohberg system also obtains all spectral information from a single point reading, which does not therefore take into account the distribution of color throughout the material. Additionally, the Hohberg system utilizes a "Flash Lamp" which does not provide a continuous light which the present inventors consider should be used to measure the phosphorescence and fluorescence of gemstones.
Other prior art that is relevant is more fully disclosed in an abandoned U.S. patent application submitted by Kevin Valente (Ser. No. 07/884,781, filed May 15, 1992). A color camera is used to obtain tri-stimulus color information of an imaged gemstone. It has been determined that the collected data of the system is not capable of completely describing the color of objects outside the RGB(Red-Green-Blue) color space. A continuous light source is used which provides a single lighting angle for analysis.